Transcript chapter02-I - Civil & Environmental Engineering

THE ENGINEERING OF FOUNDATIONS
A BOOK BY RODRIGO SALGADO
POWERPOINT RESOURCES FOR
CHAPTER 2
Contains copyrighted material
THE DESIGN PROCESS
DESIGN GOALS
• Safety
• Serviceability
• Economy
The optimum foundation solution transfers the
superstructure loads to the ground in a way that
minimizes cost (construction and maintenance) over
the life of the structures without sacrificing
safety or performance
RELIABILITY-BASED
DESIGN
LIMIT STATES DESIGN
AND WORKING STRESS
DESIGN
TYPES OF LIMIT STATES
• ULTIMATE LIMIT STATES (ULS) :
states at which a potentially unsafe/dangerous condi
tion is reached (typically structural failure, collapse,
etc.)
• SERVICEABILITY LIMIT STATES (SLS) :
states at which the structure or facility ceases to p
erform as intended without loss of safety
Limit states for shallow foundation design
Limit
State
Nature of Limit State
Consequences
IA-1
Classical bearing capacity
failure (ULS)
Excessive
movement/collapse
of
foundation causes serious damage, partial
collapse or complete collapse of structure
IA-2
Structural
failure
of
foundation element (ULS)
Column is inadequately supported by
foundation element, punching through it;
this causes serious damage, partial or
complete collapse of superstructure
IB
Excessive
foundation
(ULS)
differential
settlement
Excessive differential settlements create
excessive additional loads in the structure,
leading to structural damage
II
Excessive settlement (total
or differential) (SLS)
Excessive
settlements
lead
to
serviceability problems, such as access
problems, damage to architectural
finishings, etc.
III
Stability failure of the
whole foundation system
or a subset thereof
Collapse mechanism develops that
encompass the foundations for the
building or structure or a part of the
foundations (a classical example would be
stability failure of a slope on top of which
is founded a building)
Working Stress Design
Ultimate
load
Design
load
Qult
Qa 
FS
Allowable load
Qd  Qa
Factor of Safety (FS)
All load and resistance uncertainities are
lumped in this single factor
Ultimate load: the load that would lead to an ultimate limit
state
Design or working load: the sum of the loads under
consideration (unfactored)
ULTIMATE LIMIT STATES:
LOAD AND RESISTANCE
FACTOR DESIGN
TOLERABLE FOUNDATION
MOVEMENTS
Types of Settlement
• Uniform Total Settlement
• Non-Uniform Settlement
• Tilting
Uniform Total Settlement
• No structural damage
• Damage at interface with utilities
• Access problems
• Drainage problems
Non-Uniform Settlement
• Structural
– Cracking of beams/slabs/column
• Architectural
– Cracking of panel walls
– Door/window jamming
– Uneven Floors
• Damage at interface with utility lines
• Access and drainage problems
• Excessive Tilting
Cracks Caused by Foundation
Movement
• May have any direction, depending on the direction of
the tensile strain (cracks are  to the tensile strain)
• At a specific point of the structure, cracks have the
same direction (contrast that with the web pattern
observed in the case of shrinkage and drying)
• Appear on both sides of an element
Typical Building Structures
• Load Bearing Walls
• Frames